I created a new Last.fm account (since you can't change the account name) and imported my iTunes plays. Too bad that you can only do that once. Of over 110,000 plays, almost half were on another Mac, in another iTunes library, so they were effectively lost.
I knew about the limitation, and I can live with the result, but it sure would be nice if the client would let you import from more than one iTunes library.
Importing from multiple iTunes libraries